1. Field of the Invention
The present invention relates to a variable focus prism and to an optical system using the same.
2. Description of the Related Art
There is a conventional technique that uses a prism configured from at least three surfaces being surfaces that allow transmission and surfaces that reflect light to thereby reduce the width or size of an overall optical system using the prism by bending an optical path or folding the optical path. However, use of only a normal prism does not enable variation of the focal length of the prism itself since it is a solid element. There are examples of the addition of various constituent elements to an optical system using a prism to execute zoom or focus. The optical element disclosed in Japanese Patent Laid-Open No. 2008-191647 disposes a transmissive liquid lens for varying a focal length in proximity to the incident side or emission side of a three-sided prism. The transmissive liquid lens is a lens that is subjected to electrowetting driving (EW driving) to vary the radius of curvature of the boundary surface between an electrolyte solution and a non-electrolyte solution by application of a voltage. Furthermore, the optical system disclosed in Japanese patent Laid-Open No. 2001-4809 enables further a decrease in thickness or a wider angle, in particular, by dividing the angle of view with reference to the imaged object and forming the overall component as a divided prism. This optical system disposes a plurality of units of prisms and variable mirrors (deformable mirrors) in each division of the field of view for the imaged object to thereby focus each variable mirror on each divided field of view. Furthermore, use of a reflective liquid lens has been disclosed in Japanese patent Laid-Open No. 2007-121980 in relation to a transmissive liquid lens. This reflective liquid lens is configured from a transparent liquid and a magnetic fluid that enables variation of the radius of curvature of the magnetic fluid as a result of the strength of the magnetic field applied to the magnetic fluid.
However, the liquid lens employed in the optical element disclosed in Japanese patent Laid-Open No. 2008-191647 is only transmissive. Furthermore, a light beam in the optical system disclosed in Japanese patent Laid-Open No. 2001-4809 repeats emission and incidence using a prism in the optical path. Therefore, a chromatic aberration may be produced in a refractive surface that has a refractive power (optical power). Furthermore, the optical element disclosed in Japanese patent Laid-Open No. 2008-191647 and the optical system disclosed in Japanese patent Laid-Open No. 2001-4809 dispose an optical element that enables variation of the focal length (hereinafter referred to as “variable element”) in proximity to the prism. However, when taking into consideration preventing interference between the variable element and the prism, the overall size of the optical system is increased. For example, when executing a zoom operation, at least two variable surfaces are required to function respectively as a variator and compensator. Therefore, since at least three constituent elements having one prism and two variable elements are required as an optical system, the dimensions or adjustment processing steps are increased, and therefore results in a corresponding cost increase. Furthermore, although the reflective liquid lens disclosed in Japanese patent Laid-Open No. 2007-121980 is controlled by use of a magnetic field control method on the magnetic fluid, practical application is not enabled since the mechanism used for magnetic control is relatively large, and response characteristics are poor.